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GRADING AND SCREENING 245
W. Cross show pierced plates with a step surface produced by bending or rolling the intent being apparently to employ such plates as stationary inclined screens and obtain a gentle rolling or turning over effect for friable material such as coal. Huge shaking riddles suspended by rods have the preference in coal breakers over revolving screens because of le'ss tumbling about and breakage on this account.
The use of the flat or inclined shaking screen is indicated where precise work is desired, for loose dry non-coherent material not too finely divided and for high specific gravity. In grading sulphur, reels of special design are employed. Sulphur agglomerates so readily that it is frequently ruined after grading by indiscreet handling of the barrels in which it is shipped.
Gyrating screens are used on light finely divided materials such as flour stock where, owing to extreme fineness, consequent development of cohesion and adsorption, interstitial effect is nil, also with materials such as cork with which owing to the low specific gravity does not stratify well. On slippery and perfectly cleavable material such as mica and flake graphite gyratories find favor for each revolution of the machine tends to move the grains in a short spiral which while it spreads them well, does not afford them the opportunity of moving in erratic paths with speed too high to give much opportunity to pass through the screen openings.
In overcoming the forces of cohesion and adsorption which tend to make the grains act as a buoyed up mass it is necessary to impart a motion which will make the bed of material as thin as possible and provide numerous impacts of the grams to keep them apart and overcome the force of cohesion. On a flat horizontal gyrating screen the grains tend to move in circles. If the screen be slightly inclined the grain will tend to pursue a spiral path towards the discharge edge but its actual path is much modified by numerous collisions with other grains. Also as must be evident the long path of the stock in contact with the bolting cloth affords the time and means of breaking up its coherence and of "rubbing it through." In material such as flour stock the bulk of the grains are very finely divided and if the coherence be overcome while in contact with the screen their elimination is theoretically very simple. In bolting, blinding offers the chief obstacle to rapid screening and all gyratory bolters are provided with chains or discs or other means for overcoming blinding.
Blinding is so serious in screening damp finely divided materials such as clay that vibrating screens are preferred on this class of material to gyratory machines. On comparatively coarse loose material such as abrasives where a light feed would be desirable to secure precision of work a gyratory screen would give good results but the objection to its use on this kind of material would be the excessive wear of the screen cloth.
Wet Screening with Flat Shaking Screens.—Where manufacturing or cleaning is done in water it is often advantageous to screen in water. The principal advantageous effect of wet screening is the rapid and direct transportation of the undersize grains through the openings but as will be shown a little later this action can only be secured imperfectly. The other advantages of wet screening are the washing off of coats of dust adhering to the grains and a cheap and simple mode of transporting oversize and undersize products to and from the screens and with less head room than would be required with dry spouts.
In hand screening in a suitable vessel to hold the undersize grains which pass through the screen and the water aiding in the screen operation the thing noticed at once is that unless the mass on the screen is stirred occasionally with the fingers little or no screening action takes place. Again if the screen is pushed down too deeply in the water it is seen that the effect is not so good as where it is pushed down